This should do the trick:

declare @is_thanksgiving bit,
    @cur_date datetime = getdate()

select @is_thanksgiving = case when datename(month, @cur_date) = 'November'
        and datename(weekday, @cur_date) = 'Thursday'
        and day(@cur_date) between 22 and 28
    then 1 else 0 end

The 4th Thursday should be between:

• the 22nd (if the first day of the month is a Thursday...early Thanksgiving) , and
• the 28th (if the first day of the month is a Friday...late Thanksgiving)

This is very similar to Christian Pena's answer, but I think his is off by 1.